This invention relates to the field of smoke detectors, and more particularly to photoelectric type smoke detectors.
Smoke detectors are known safety appliances which are installed in various structures, including homes, office buildings, and warehouses, to monitor for the presence of smoke, and to provide an alarm in the event smoke is detected. The alarm may be audible and/or visual within the monitored space, and may be electronically communicated to a remote monitoring site.
While the performance feature offerings among different vendor model smoke detectors may vary, actual smoke detection is usually based on either an ionization or a photoelectric detection technology. Ionization smoke detectors are more sensitive than the photoelectric type detectors in detecting smaller particles of combustion, i.e. generally smaller than one micron (considered generally invisible to the human observer), which are predominately created by fast flaming fires. Alternatively, photoelectric smoke detectors are more sensitive than ionization detectors in detecting large combustion particulate, i.e. generally larger than one micron (considered to be visible to the human observer), which are created by smoldering fires. Common among both types of detectors, however, is that their chambers must be open to the environment, since they each measure the physical, not the gaseous, products of combustion.
The open chambers subject each type of smoke detector to ambient air quality conditions, including dust. Dust is a minor source of error for ionization detectors, which use a radioactive source to ionize the air molecules flowing into the chamber and an applied electric field to force the ionized air molecules to flow from the radioactive source to the detector""s electrically conductive housing. The ionization detector""s sensitivity is fixed by the geometry of the chamber and the internal sensing structure. However, dust and other similar physical debris more seriously affect the performance of photoelectric smoke detectors, which are optical devices.
Photoelectric detectors include an infrared (IR) light source and an IR photodiode receiver positioned at opposite ends of the detector""s chamber. They are located off axis from each other to prevent the IR light source emitted energy from flowing directly to the receiver. Light absorbing baffles and coatings within the chamber are used to attenuate all quiescent state IR reflections, to provide a controlled, minimum value of photodiode current in the non-smoke state. In the event of a fire, combustion particles entering the detector""s chamber disturb the quiescent state absorption characteristics, thereby producing IR scattering and causing IR energy to be detected by the photodiode. The photodiode responds by providing an output electrical current at a magnitude proportional to the detected IR, and when the current exceeds a selected threshold the detector sounds the alarm.
The magnitude of the output current provided by the photodiode is directly proportional to the intensity of the scattered IR it receives, which in turn is generally directly proportional to the density of the combustion particles entering the chamber. Using well established standards, the photoelectric detector""s alarm threshold is correlated to a given level of smoke density by calibration to an associated magnitude of photodiode output current. With knowledge of the photoelectric detector""s optical scattering characteristics and the offset tolerances of the IR source and photodiode, this calibration process ensures accurate, repeatable performance. Once installed, however, it is the ambient optical and electrical noise within the monitored space that sets a xe2x80x9cnoise floorxe2x80x9d within the chamber, which manifests itself in terms of a quiescent xe2x80x9coffsetxe2x80x9d current at the photodiode output.
The photoelectric chamber of prior art photoelectric smoke detectors is made of plastic, which may be injection molded into the fine details required in the chamber structure. Due to the presence of the active IR source and photodiode within it, the plastic material is also dielectric, to prevent the possibility of electrical shorting of the electrical component leads to the case. The dielectric material is therefore an insulator, which prevents electron flow through it.
As normally occurring air currents flow into the chamber, the friction of the air molecules across the interior surfaces of the chamber""s dielectric material housing induces an electrostatic charge in the housing through the known triboelectric effect. This electrostatic charged surface is capable of attracting and holding dust fibers and similar microscopic debris flowing near its surface. In effect, this dielectric material chamber functions in a manner similar to an electrostatic air collector, or filter.
As dust particles enter the photoelectric chamber and settle on or are attracted to its internal surfaces, they build up a coating which modifies and in time alters the light absorbing properties of the black surfaces of the inner surfaces of the chamber housing walls. This in turn alters the chamber""s absorption and scattering characteristics, resulting in scattered IR energy being reflected back to the photodiode as noise in either the clean air state or during smoke conditions. As the surface dust levels build, the noise increases, decreasing the detector""s signal-to-noise ratio. Since the alarm threshold is calibrated to a given level of receiver signal magnitude, the signal build-up due to the additional noise effectively decreases the concentration of combustion particulate necessary to establish an alarm condition. This may result in the sounding of false, or premature alarms.
It is for these reasons that manufacturers of photoelectric smoke detectors recommend that the detector, including the photoelectric chamber, be kept as clean as possible by periodic cleaning to remove the dust build-up. This includes either washing and/or vacuuming the chamber. However, due to the unobtrusive nature of the detector this maintenance cleaning is often forgotten. This is especially true in residential applications where there are no established maintenance regimes, as there may be in industrial applications. It is desirable, therefore, to provide a photoelectric detector in which the effect of dust build-up on detector performance is minimized.
One object of the present invention is to provide a photoelectric smoke detector which is dust tolerant in its performance characteristics. Another object of the present invention is to provide a photoelectric smoke detector with lower maintenance requirements. Still another object of the present invention is to provide a photoelectric smoke detector with higher reliability performance.
According to the present invention, a photoelectric smoke detector for monitoring the ambient air within a space, includes an assembly having a base unit and a cover unit adapted to releasably engage each other to form an enclosure which is capable of receiving therein the ambient air of the selected space, the photoelectric smoke detector further including a voltage signal source and a photoelectric chamber disposed within the enclosure, the chamber having a housing comprising an electrically conductive material, with a plurality of baffles disposed along a surface thereof to permit ambient air to enter the chamber without inducing an electrostatic charge in the housing, the chamber further having disposed therein a light source for emitting light energy and a light receiver for providing an output signal at a signal magnitude which is proportionate to the intensity of its received light, the light source, the light receiver, and the baffles being arranged within the chamber housing to provide a minimum magnitude output signal in a quiescent state identified as the absence of constituents of combustion in the ambient air.
In further accord with the invention, the electrically conductive material housing of the photoelectric chamber comprises a metal. In still further accord with the present invention, the chamber housing material is of a conductive thermoplastic material comprising a chemical compound of plastic resins and one or more of a variety of conductive filler materials. In yet still further accord with the present invention, the conductive filer material is selected from among the group consisting of carbon black, carbon fiber, metal fiber, metal-coated carbon fiber, and metal powders.
These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying Drawing.